apache/druid · error · org.apache.druid.java.util.common.IAE

Dimension selector is currently unsupported for

Error message

Dimension selector is currently unsupported for [%s]

What it means

VariantColumn.makeDimensionSelector refuses to build a DimensionSelector when the column has no known variant types (variantTypes == null) and its logical type is an array. Array-only variant columns must be accessed through array/object selectors, not the dimension-selector API, so the column throws IAE immediately.

Solutions

Use UNNEST or SQL array functions (ARRAY_CONTAINS, ARRAY_OVERLAP, etc.) instead of grouping on the raw array
Use a native array filter (filter selector) rather than a dimension selector
Ensure the column's type info was ingested so variantTypes is populated for mixed-type columns
Rewrite the native query to use an object/vector selector path for the array column

Example fix

// before
DimensionSelector sel = column.makeDimensionSelector(offset, extractionFn); // array column
// after
Object val = objectSelector.getObject(); then use array-aware processing or UNNEST in SQL

Defensive patterns

Strategy: validation

Validate before calling

if (column.getLogicalType().isArray()) { /* do not create a dimension selector; use array/object selector */ }

Type guard

static boolean canMakeDimensionSelector(VariantColumn c) { return !(c.getLogicalType().isArray()); }

Try / catch

try { sel = column.makeDimensionSelector(offset, fn); } catch (IAE e) { if (e.getMessage().contains("Dimension selector is currently unsupported")) { /* fall back to object selector */ } else { throw e; } }

Prevention

Never group or filter directly on ARRAY-typed nested columns via dimension specs
Use UNNEST/array SQL functions for arrays
Ensure ingestion records variantTypes so mixed-type columns get selector support

When it happens

Trigger: Calling makeDimensionSelector (via dimensionSelector) on a variant column whose logicalType is an array (e.g. ARRAY<STRING>) and whose variantTypes was not populated, e.g. from a query plan or cursor that selects the array column as a group-by dimension.

Common situations: Group-by or dimension-filtered queries referencing an ARRAY column directly; native queries using a dimension spec on an array-typed nested field; SQL that doesn't apply UNNEST or array functions before grouping.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/2caed8c57cb277cb. Report an issue: GitHub.

Appendix: source

Thrown at processing/src/main/java/org/apache/druid/segment/nested/VariantColumn.java:382

  @Override
  public int getCardinality()
  {
    if (logicalType.isArray() && variantTypes == null) {
      return arrayDictionary.size();
    }
    // this probably isn't correct if we expose this as a multi-value dimension instead of an array, which would leave
    // the array dictionary out of this computation
    return stringDictionary.size() + longDictionary.size() + doubleDictionary.size() + arrayDictionary.size();
  }

  @Override
  public DimensionSelector makeDimensionSelector(
      ReadableOffset offset,
      @Nullable ExtractionFn extractionFn
  )
  {
    if (variantTypes == null && logicalType.isArray()) {
      throw new IAE("Dimension selector is currently unsupported for [%s]", logicalType);
    }
    // copy everywhere all the time
    class StringDimensionSelector extends AbstractDimensionSelector
        implements SingleValueHistoricalDimensionSelector, IdLookup
    {
      private final SingleIndexedInt row = new SingleIndexedInt();

      @Override
      public IndexedInts getRow()
      {
        row.setValue(getRowValue());
        return row;
      }

      public int getRowValue()
      {
        return encodedValueColumn.get(offset.getOffset());
      }

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